This invention relates to an SDH (synchronous digital hierarchy) network. Communication systems using optical networks, and to a lesser extent radio networks are increasingly employing digital communication patterns. The European systems are being standardized by the CCITT on SDH whereas the American systems are known as SONET (synchronous optical networks) and operate on the ANSI standard. The SDH system uses a network-node interface and is compatible with SONET. With the network node system it is necessary to drive information around a network using a clock pulse and this is normally obtained from an external timing source. The invention is applicable to any such system.
Within such a network it is important that the clock pulse used within a node for synchronization purposes is of as high a quality as possible--i.e. that it has not deteriorated to the extent that there is uncertainty in its value. One problem where such deterioration can occur is when a closed timing loop forms such that the same timing signal is sent repeatedly around a loop and where deterioration occurs in the clock signal each time it passes through a node. It is one object of the invention to provide a means for preventing the occurence of such closed timing loops.